Precise solid-phase synthesis of CoFe@FeOx nanoparticles for efficient polysulfide regulation in lithium/sodium-sulfur batteries

Complex metal nanoparticles distributed uniformly on supports demonstrate distinctive physicochemical properties and thus attract a wide attention for applications. The commonly used wet chemistry methods display limitations to achieve the nanoparticle structure design and uniform dispersion simultaneously. Solid-phase synthesis serves as an interesting strategy which can achieve the fabrication of complex metal nanoparticles on supports. Herein, the solid-phase synthesis strategy is developed to precisely synthesize uniformly distributed CoFe@FeOx core@shell nanoparticles. Fe atoms are preferentially exsolved from CoFe alloy bulk to the surface and then be carburized into a FexC shell under thermal syngas atmosphere, subsequently the formed FexC shell is passivated by air, obtaining CoFe@FeOx with a CoFe alloy core and a FeOx shell. This strategy is universal for the synthesis of MFe@FeOx (M = Co, Ni, Mn). The CoFe@FeOx exhibits bifunctional effect on regulating polysulfides as the separator coating layer for Li-S and Na-S batteries. This method could be developed into solid-phase synthetic systems to construct well distributed complex metal nanoparticles.

Intensity(a.u.) Raman shift(cm -1 ) CoFe@FeO x CoFe@C I D /I G =1.01 3 Figure S4 The STEM image and corresponding EDS elemental mapping images of the CoFe@FeOx.

Figure S5
The TEM image of the CoFe@FeOx with the size distribution histogram (The size distribution is obtained from TEM images analysis by using at least 100 NPs.).S10 Characterization of CoFe@FeOx.a χ(R) space spectra fitting curve and b the inverse FT χ(R) space spectra into χ(q) space spectra (χ(k) space) fitting curve of the CoFe@FeOx at Fe K edge.c χ(R) space spectra fitting curve and d the inverse FT χ(R) space spectra into χ(q) space spectra (χ(k) space) fitting curve of the CoFe@FeOx at Co K edge.

Figure S17
The segregation energy of Fe and Co atoms for the CoFe alloy in the absence and presence of CO adsorption, and the corresponding structures are presented in Figure S16.

In fact, previous studies have confirmed that B2 CoFe alloy prepared by the mechanical alloying
2][3][4][5] For example, Mizuno et al. 1 found that B2 CoFe alloy existed in a wide range of Fe composition from 30 to 75 at% at 773 K, however, aiming at compensating the deviation from the stoichiometric composition, constitutional defects are introduced; the formation energies of the vacancy and antisite defect in the CoFe alloy with different Fe compositions showed that both the vacancy and antisite defect are prone to be formed in the CoFe alloy with 50 at% Fe.Meanwhile, the chemically synthesized B2 CoFe nanoparticle also has defect owing to its synthesis method of one pot polyol process using ethylene glycol as a reducing agent, resulting in the disordered nature. 2 Moreover, the vacancies are also experimentally observed in the FeCo alloy, and the self-diffusion of the metals in both the disordered (A2) and ordered (B2) phase CoFe alloy occurs through the vacancies. 3Furthermore, Fu et al. 4 theoretically studied the structural stability, point defects and order-disorder transition of B2 CoFe alloy, suggesting that B2 CoFe alloy is marginally stable, weakly ordered with a high density of antisite defects.7][8] For example, Kim et al. 6

Commercial multiwalled CNTs and S powder composite is employed as cathode, in which active
S accounts for 70 wt% in the composite cathode as shown in Figure S21.

Figure S22
The optical photographs of the commercial PP separator and CoFe@FeOx/PP separator.
The CoFe@FeOx/PP obtained via the tape casting exhibits smooth surface according to the optical photographs in Figure S22, which is similar to that of commercial PP separator.

Detailed performance comparisons of Li-S batteries with traditional PP separator, CoFe@C/PP
and CoFe@FeOx/PP are exhibited in Figure S24a.The Li-S battery with CoFe@FeOx/PP can maintain a high reversible capacity of 913 mAh g -1 after 100 cycles.As a contrast, a low initial capacity of 595 mAh g -1 is obtained with the traditional PP separator, demonstrating the CoFe@FeOx can effectively anchor LIPSs and facilitate the conversion of sulfur species.And the Li-S battery with the CoFe@C/PP exhibits an initial reversible capacity of 993 mAh g -1 at 0.2 A g -1 and maintain a reversible capacity of 631 mAh g -1 after 100 cycles, which is poorer than that of the CoFe@FeOx based battery.The rate performance comparison of batteries with the CoFe@FeOx/PP and CoFe@C/PP are showed in Figure S24b.The CoFe@FeOx based battery can exhibit high specific capacities of 1376, 927, 799, 706, 594 and 536 mAh g -1 at 0.2, 0.5, 1, 2, 5 and 8 A g -1 , respectively.However, the battery with CoFe@C/PP could only deliver specific capacities of 957, 675, 612, 549, 451 and 358 mAh g -1 at 0.2, 0.5, 1, 2, 5 and 8 A g -1 , respectively.The long-term cycling performance with the CoFe@FeOx/PP is evaluated at 1 A g -1 as shown in Figure S24c  The high areal S loading electrode is fabricated to further check practical application of Li-S batteries with the CoFe@FeOx/PP (Figure S25a and S25b).Even with high sulfur loading mass of 6.8 mg cm -2 , the CoFe@FeOx based battery can achieve a high initial capacity of 1294 mAh g -1 and maintain a superior reversible capacity of 752 mAh g -1 after 100 cycles at 0.1A g -1 , indicating the desirable application prospect of CoFe@FeOx for boosting high energy density practical Li-S batteries.The morphologies of Na anode with commercial GF separator and CoFe@FeOx/GF after three cycles at 0.2 A g -1 are displayed in Figure S35a and S35b.Na anode exhibits rough and corroded surface in Na-S battery with the commercial GF separator.When used the CoFe@FeOx/GF separator, the Na anode presents relatively smooth surface, further demonstrating the CoFe@FeOx/GF separator could inhibit the shuttle effect and protect the Na anode.

Figure
Figure S1 Characterization of Co-Fe PBA and CoFe@C.a XRD patterns of Co-Fe PBA with various pyrolysis temperatures.b The TEM image of the CoFe@C with the size distribution histogram (The iron size distribution is obtained from TEM images analysis by using at least 100 NPs.).

Figure
Figure S3 Characterization of CoFe@FeOx.a The STEM image and b EELS spectra from locations marked as core, shell and blank of the CoFe@FeOx.

Figure
Figure S6 Characterization of CoFe@FeOx.a TEM and b HRTEM images of CoFe@FeOx with syngas treatment temperature at 300 °C.

Figure
Figure S7 Characterization of CoFe@FeOx.a TEM and b HRTEM images of CoFe@FeOx with syngas treatment temperature at 500 °C.

Figure S9
Figure S9XPS files of the CoFe@C and CoFe@FeOx.

Figure
Figure S11 Characterization of CoFe@FeOx.The experimental and fitted data of a Fe and b Co K-edge EXAFS of the CoFe@FeOx.

Figure 5 Figure
Figure S12 Co K edge WTEXAFS of reference Co foil, CoO, and Co2C, and Fe K edge WTEXAFS of reference Fe foil, Fe2O3, and χ-Fe5C2.

Figure S14
Figure S14The GC-FID results of the exhaust during carburization.

Figure S16
Figure S16 The side and top views of CoFe (110) surface.The blue and purple color balls are Co and Fe atoms, respectively.

9
Figure S18 The structures of Fe and Co atoms segregation for the CoFe alloy in the absence and presence of CO adsorption.The blue, purple, grey, and red color balls are Co, Fe, C and O atoms, respectively.The bright yellow represents the Fe or Co atom located in the n th layer.

Figure S19
Figure S19 The segregation pathway and reaction energy profile of Fe or Co atom in the CoFe alloy.The segregation pathway of a Fe atom and b Co atom in the CoFe alloy with the Co vacancy located in the 2 nd layer.c The reaction energy profile of the Fe or Co atom segregation pathway in the absence of CO adsorption.d The reaction energy profile of the Fe or Co atom segregation pathway in the presence of CO adsorption.IS, IM and FS represent the initial, intermediate, and final state, respectively.The blue and purple color balls are Co and Fe atoms, respectively.And the corresponding structures are presented in Figure S18.
the surface Au atoms contribute to the selective production of CO; meanwhile, aiming at analyzing the reconstruction of NiAu alloy surface, DFT calculations are adopted to consider the segregation pathway of Ni in the NiAu alloy, then, the model of NiAu alloy is constructed, in which an Au vacancy was set initially at the second atomic layer owing to the easy formation of Au vacancy, and the possible segregation pathway for a third-layer Ni atom near the vacancy was investigated to change the position of Ni through a series of the exchange steps between Au/Ni atom and the vacancy exchange steps.Based on above analysis, the presence of vacancies in the CoFe alloy was confirmed, meanwhile, similar to above reported studies byZhang et al.,8 in our present study, a Fe or Co atom vacancy in the CoFe alloy was set initially at the second atomic layer, then, the possible pathway for a thirdlayer Fe or Co atom near the vacancy was proposed to change the position of Fe or Co atom through a series of the exchange steps between Fe/Co atom and the vacancy, which could realize Fe/Co atom segregation.

Figure
Figure S20 The Fe and Co atoms segregation pathway in the CoFe alloy.The structures of Fe and Co atoms segregation pathway in the a absence or b presence of CO adsorption in the CoFe alloy with Co vacancy located in the 2 nd layer.IS, IM and FS represent the initial, intermediate, and final state, respectively.The blue, purple, grey, and red color balls are Co, Fe, C and O atoms, respectively.The bright yellow represents the vacancy located in the n th layer.The bright blue represents the M (M=Fe or Co atom) located in the n th layer.

Figure S21
Figure S21 The commercial multiwalled CNTs and S powder composite cathode (S content: 70 wt%).
Figure S23 Electrochemical tests of Li-S batteries with CoFe@FeOx/PP with 70 wt% of sulfur loading in the cathode.a Typical charge/discharge curves at 0.2 A g -1 .b Typical cyclic voltammogram (CV) curves at 0.2 mV s -1 .The charge/discharge curves of Li-S battery are shown in FigureS23a.The battery with CoFe@FeOx/PP can deliver a high initial capacity of 1537 mAh g -1 (around 92% of theoretical value) at 0.2A g -1 .According to CV curves at 0.2 mV/s (FigureS23b), two typical cathodic peaks can be assigned to the sequential reduction of S8 to soluble LiPSs and further convert to solid Li2S2/Li2S.The charge plateau at 2.23-2.42V is related to oxidation reaction of Li2S to LiPSs and S8.

Figure S24
Figure S24 Electrochemical performance with different separators.a Cycling performance comparison of Li-S batteries with different separators with 70 wt% of sulfur loading in the cathode at 0.2 A g -1 .b Rate capability comparison of CoFe@FeOx/PP and CoFe@C/PP based Li-S batteries with 70 wt% of sulfur loading in the cathode at various current densities.c Long-term cycling life of CoFe@FeOx/PP based Li-S batteries with 70 wt% of sulfur loading in the cathode at 1 A g -1 .The , which could deliver a long lifespan around 1000 cycles.The superior electrochemical performance suggests that the CoFe@FeOx with abundant polar active sites in the FeOx shell and highly conductive CoFe alloy core can significantly suppress the shuttling of polysulfides and promote the conversion process of LIPSs.

Figure S25
Figure S25 Electrochemical performance of CoFe@FeOx/PP based battery with high sulfur loading of 6.8 mg cm -2 at 0.1 A g -1 .a Cycling performance.b Corresponding charge/discharge curves This battery is composed of Na metal anode and CNTs and S composite cathode with 70 wt% of sulfur loading.

Figure
Figure S26 CV curves and corresponding linear fitting plots.CV curves of a CoFe@FeOx/PP and b CoFe@C/PP based Li-S batteries with 70 wt% of sulfur loading in the cathodes at various scan rates.The linear fitting plots of c CoFe@FeOx/PP and d CoFe@C/PP based batteries.As shown in Figure S26a-d, the reaction dynamics comparisons between the CoFe@FeOx and CoFe@C based batteries are analyzed via the power-law equation (log(i) = log(a) + blog(v), i and v are the peak current and scanning rate and a and b are the adjustable parameters). 23Both the cathodic peak and anodic peak with the CoFe@FeOx based battery show higher b values than those of the CoFe@C based battery, demonstrating much faster reaction kinetics with the CoFe@FeOx/PP.

Figure S28 The
Figure S28 The Li2S precipitate experiments for Li-S batteries.a CoFe@FeOx.b CoFe@C.

Figure S30
Figure S30The commercial multiwalled CNTs and S powder composite cathode (S loading: 50 wt%).In view of more slower reaction kinetics derived from larger Na ion radius and more severe volume expansion issues of Na-S batteries than those of Li-S batteries, the CNTs and S composite cathode with S content of 50 wt% is attempted to assemble Na-S batteries.The CNTs and S composite cathode with good dispersion is shown inFigure S30.

Figure S31
Figure S31The typical CV curves of Na-S battery with CoFe@FeOx/GF with 50 wt% of sulfur loading at 0.2 mV s -1 .

Figure S34
Figure S34Cycling performance of CoFe@FeOx/GF based Na-S batteries at 2 A g -1 .This battery is composed of Na metal anode and CNTs and S composite cathode with 70 wt% of sulfur loading.

Figure S35
Figure S35 Morphology analysis.The morphologies of Na anode for Na-S batteries with 50 wt% of sulfur loading with a commercial GF separator and b CoFe@FeOx/GF separator after three cycles at 0.2 A g -1 .

Figure
Figure S36 Cycling performance with different separators.Cycling performance of aMnFe@FeOx/GF and b NiFe@FeOx/GF based Na-S batteries at 0.2 A g -1 .The batteries in Figure

Table S1
Mössbauer parameters of the CoFe@C and CoFe@FeOx concluded the presence of vacancies in the CoFe alloy based on ab initio statistical mechanics.Above these previously reported studies showed the presence of vacancies in the CoFe alloy, as a result, in our present study, B2 CoFe alloy is employed to explore its Fe segregation, in which B2 CoFe alloy with the vacancy is considered.On the other hand, the vacancies in the CoFe alloy cannot be well characterized experimentally in our studies, however, our experiment results showed that Fe atoms would aggregate on the CoFe alloy surface, which means that Fe atoms easily segregates from the bulk to the surface in the CoFe alloy, however, in order to take place the segregation of Fe atoms from the bulk to the surface, only the presence of vacancies in the CoFe alloy could initiate Fe segregation, and realize the segregation of Fe atoms from the bulk to the surface.Moreover, theoretical calculation models of CoFe alloy also further verified that only the presence of vacancies in the CoFe alloy could realize the occurrence of Co/Fe segregation pathway from the bulk to the surface.
[6][7][8][6][7][8].Our results show that the formation energies of Fe or Co vacancy initially set in the 2 nd layer with the absence of CO adsorption are 5.5 or 2.0 eV, namely, the formation of Co vacancy in the 2 nd layer for CoFe alloy more easily occurs compared to that of Fe vacancy.Further, compared to the CoFe alloy surface in the absence of CO adsorption, CO adsorption promotes the formation of Fe or Co vacancy in the 2 nd layer for CoFe alloy, however, 1, Eseg-2 and Eseg-3 of Fe atom for CoFe alloy are 4.75, 4.13 and 2.31 eV, respectively, suggesting that the ability of Fe atom segregation from the bulk to the surface is weaker.Meanwhile, the Eseg-1, Eseg-2 and Eseg-3 of Co atom for CoFe alloy are -2.54,-1.80 and -5.71 eV, respectively, which means that the Co atom segregation from the bulk to the surface for CoFe alloy is much easier compared to the Fe atom segregation in the absence of CO adsorption.In the presence of CO adsorption, CO adsorption alters the Eseg-n(n=1-3) of Fe and Co atoms for CoFe alloy.preferentially carburized to form a FexC shell on the CoFe alloy.Aiming at further analyzing the preference of Fe or Co atom segregation in the CoFe alloy, the reaction energy of Fe or Co atom segregation pathway is calculated.As mentioned above, in general, the alloy segregation takes place with the exchanges between the metal ions and surface/subsurface the formation energy of Co vacancy is lower than that of Fe vacancy (-16.0 vs. -14.3eV), namely, the formation of Co vacancy is much easier than that of Fe vacancy.Above results show that the formation of Co vacancy for the CoFe alloy is easier than that of Fe vacancy irrespective of CO adsorption, thus, only the Co vacancy in the 2 nd layer for CoFe alloy is considered in our study.